Former US senator George Mitchell, who had travelled to Northern Ireland to help broker the talks, warned that the hard-fought negotiations were likely to pale in comparison to the challenge of putting the agreement into practice.

“(He) said to myself and Martin (McGuinness), he may have said it to everybody else as well, ‘Well, that’s the easy bit done; the hard bit’s implementing it’. And he was absolutely right,” said the Louth TD, as he spoke from his office in Leinster House in Dublin.

The months and indeed years leading up to the final agreement had been spent preparing the grassroots, said the 69-year-old, who retired as Sinn Fein party president in February after 34 years at the helm.

“In terms of the strategy and the general thrust of what we were trying to do we kept very, very close to our activists and to the wider republican constituency.”

He added: “It wasn’t just something that came out of the blue, it was part of a continuum.”

After that, he and Mr McGuinness faced down threats from dissidents opposed to the peace process, a difficult but necessary stance to take, he said.

“We wouldn’t give them an inch. And that was a dangerous thing to do and probably still remains a dangerous thing to do.”

He rejected criticism that the agreement, amid the absence of government in Northern Ireland following the collapse of powersharing and the uncertainty around the Irish border issue, may have had its day.

It “doesn’t pretend to be a settlement”, he said. “It is an agreement on a journey, without agreement on the destination.”

Governance by way of anything other than mandatory coalition could only happen in a “new Ireland”, he said, speaking confidently of the party’s ultimate aim of all-island unity.

“Until we get to the destination the only arrangement that I think that can work is powersharing, based on tolerance, based on the principles of the Good Friday Agreement, and that includes the all-Ireland architecture.”